Solid-state Raman lasers are a practical and efficient approach to optical frequency down conversion, offering high (up to 70 to 80%) conversion efficiencies with respect to the pump power, excellent beam quality and ease of alignment. In recent years use of crystals for stimulated Raman scattering (SRS) has been gaining interest because, in comparison with high-pressure gaseous and dye (liquid) Raman lasers, crystalline Raman lasers offer better gain, better thermal and mechanical properties, and the ability to operate at high pulse repetition frequency. Further they are compatible with compact all solid-state laser technology.
Solid-state lasers are commonly used in the opthalmological and dermatological fields. For these applications there is commonly a need to have available a range of different wavelengths.
U.S. Pat. No. 4,165,469 (Ammann, 1979) revealed a solid-state laser capable of providing different frequencies of laser output light. The laser disclosed in U.S. Pat. No. 4,165,469 is limited to the use of lithium iodate crystal, which performs the functions both of Raman-shifting and of frequency doubling to generate a plurality of possible output frequencies based on the frequency-doubled first, second or higher order Stokes stimulated Raman scattering in the lithium iodate crystal. This limitation is a significant disadvantage, as the laser of the invention is limited to the output frequencies obtainable using lithium iodate, since it is rare to find crystals capable of performing both of these functions together. A further disadvantage is that, since the lithium iodate crystal serves two discrete functions, it is not possible to optimise the position of that crystal independently for two functions. Yet a further disadvantage is that lithium iodate has limited utility in high power applications, since it has a relatively low damage threshold. Yet another disadvantage is that, since the laser of the prior art requires a reasonably long Raman crystal (in order to obtain sufficient gain), and this Raman crystal needs to be rotated in order to achieve phase matching, there will be substantial beam displacement due to refraction when the crystal is off normal incidence. This means that the cavity of the prior art will need to be significantly realigned for each wavelength, which thus greatly reduces the utility of the laser of the prior art.
There is therefore a need for a versatile solid-state laser system wherein the output may be selected from two or more different wavelengths of output laser light, and which is capable of being optimised with respect to generation of Raman wavelengths and independently with respect to subsequent conversion (for example frequency doubling or sum frequency generation) of the Raman wavelength. There is a further need for such a system to be designed so that it is straightforward to manufacture, can be incorporated into a practical device without undue difficulty, and so that the step of selecting a wavelength is easily carried out by the operator or end-user of the system.